In electrical discharge machining, hereinafter referred to as EDM, conductive workpieces are machined by passing electroerosive power pulses between a workpiece electrode and a tool electrode spacedly juxtaposed therewith across an interelectrode gap flooded with a liquid machining medium which is at least in part of dielectric nature and which also serves to carry away the detritus or gap products of the electrical discharge machining process.
In a variety of EDM operations in which a conductive workpiece is drilled, shaped or cavity-sunk with a shaped tool electrode or contour-cut with a continuous, thin wire or like elongate tool electrode, a discrete electrical power pulse of a duration generally in the range between 10.sup.-7 and 10.sup.-2 seconds may be applied across a relatively carefully dimensioned machining gap with a gap size, for example, of 0.05 to 0.005 mm, and filled with the liquid machining medium to initially produce a dielectric breakdown of the gap medium across the distance of the lowest dielectric strength, that is generally the smallest distance, between a point on the tool electrode surface and a point on the workpiece electrode surface. The dielectric breakdown results in a spark discharge or a discharge of short arc type whereby a highly concentrated discharge current flows through the breakdown path between these electrode points. The applied electrical energy is highly concentrated (generally exceeding 10.sup.5 watts/cm.sup.2 with a current density of 10.sup.4 to 10.sup.9 amp/cm.sup.2) and is localized within a narrow discharge column so formed. A rapid temperature rise in the discharge column brings the metal in the region of the discharge spot on the workpiece electrode surface into molten and evaporated states. At the same time a rapid expansion of the metallic gases and liquid-decomposed gases in the discharge column brings about an intense pressure (estimated to amount to 100 to 500 Kg/cm.sup.2) which effectively serves to impulsively pulverize the molten metal into fine particulate forms and scatter the particles away from the discharge spot. As a result, a crater-like formation comprising a crater recess and a surrounding crater mound is formed on the portion of the workpiece surface on which the discharge impinges. The next time-spaced pulse may then seek another pair of points on the tool and workpiece electrode surfaces and bridge across them a further high-energy electroerosive discharge. Thus, a succession of time-spaced and discrete power pulses applied across the opposed electrode surfaces provide randomly localized material removal discharges which produce cumulatively overlapping craters in the workpiece surface; the total surface is thus machined uniformly over the parts thereof confronting the tool electrode and the machined portion receives a configuration, e.g. a cavity or groove, conforming to the shape of the tool electrode.
The latter may be formed with the desired configuration of the cavity or the shape complementarily desired in the workpiece. Alternatively or in addition, the tool electrode is displaced relative to the workpiece electrode to form a desired electroerosively formed contour in the latter. During the machining operation, small metal or conductive chips or particles and other gap products (i.e. tar and gases) are carried away from the gap by the liquid machining medium which floods the gap and is generally circulated therethrough, while the tool electrode may be advanced relative to the workpiece by a servo mechanism designed to maintain a predetermined gap spacing or designed to approach the desired gap spacing as accurately as possible.
Each individual EDM crater is generally circular in shape as diagrammatically depicted in FIG. 1(a) as seen from above. The crater has the region of a crater recess A surrounded by an annular crater mound B as seen from its side elevation or sectional view shown in FIG. 1(b). The crater recess A is shown to have a depth H and a diameter Dx and the crater mound B to have a height h. It can be seen that the size of the crater is related to the amount of stock removal per discharge and hence influences both the surface roughness and removal rate of an EDM process. On the other hand, it is known that the energy of each single discharge determines the size of the resulting crater. Thus, the greater the single discharge energy, the greater becomes the stock removal and hence the removal rate but at the same time the greater becomes the surface roughness. In other words, while it is desired to use a greater energy of single discharge to achieve a greater removal rate, it has been recognized that this can be done only at a sacrifice because of a greater surface roughness that unavoidably ensues. The stock removal W per single discharge and the EDM surface roughness Rmax which results cumulative of such single discharges have been found to be correlated as follows: EQU Rmax.sup.3 .alpha.W (1)
Hence, the fact that efforts to obtain a higher removal rate and efforts to obtain a finer surface roughness (finish) are mutually incompatible has plagued the art. One has to be satisfied with a relatively poor surface finish when a high removal rate is desired or with a relatively low removal rate when a fine finish is desired.